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A comprehensive analysis of half-doped manganites is presented using MonteCarlo simulations applied to the double-exchange model with cooperativeJahn-Teller lattice distortions in two dimensions. A variety of novel resultsare reported: (i) The phase diagram is established in the lambda-J_ AF plane,with lambda the electron-phonon coupling and J_AF the antiferromagneticexchange between classical t_2g spins. The results include standard phases,such as the CE-insulating and FM-metallic regimes, but they also unveil novelstates, such as a ferromagnetic charge-ordered (CO) orbital-ordered phasecompatible with recent experimental results by Loudon et al. (ii) For realisticcouplings, it was observed that the charge disproportionation delta of the COphase is far from the widely accepted extreme limit delta=0.5 of a 3+/4+ chargeseparation. A far smaller delta appears more realistic, in agreement withrecent experiments by Garcia et al. and Daoud-Aladine it et al. (iii) Colossalmagnetoresistance (CMR) effects are found in calculations of clusterresistances using the Landauer formalism. This occurs near the ubiquitousfirst-order phase transitions between the insulating and metallic states. (iv)The CE-state is found to be (very sensitive to disorder) since its long-rangeorder rapidly disappears when quenched-disorder is introduced, contrary to theFM state which is more robust. This is also in qualitative agreement withrecent experiments by Akahoshi et al. and Nakajima et al. (v) The phase diagramin the half-doped (electron doping) regime is briefly discussed as well. Acharge-ordered state is found which is the analog of the x=0.5 CE phase. Itcontains a 3+/2+ charge arrangement at large lambda.Comment: 14 pages, 18 figures, submitted to PRB. Correction Ref. 11  (04/02/2003). Correction page 4, right column ("it was not presented...")  (05/20/2003). Fig. 2 replaced, some new text added (06/27/2003). Changes in  Ref.[7] and [24] (07/08/2003

physical review. b, condensed matter

Theoretical study of half-doped models for manganites: Fragility of CE phase with disorder, two types of colossal magnetoresistance, and charge-ordered states for electron-doped materials